1. Field of the Invention
The present invention relates to a film-forming method and apparatus which can be used to produce semiconductor devices, such as LSIs, and recording media, such as magneto-optical disks, and more particularly, to an ionization film-forming method and apparatus which can form various types of deposited films by using ionized particles.
2. Related Background Art
Film-forming methods are used to form wirings and interlayer insulating films on a variety of semiconductor devices or form magnetic and protective layers on recording media. These film-forming methods, which must exhibit various types of performance, have recently been required to provide a film having an improved coverage of an inside of a groove formed in a substrate, especially a bottom of the groove.
FIG. 5 shows a cross section of a film deposited by a conventional sputtering method. The film 102 on the bottom 104 of a groove is by far thinner than the film 100 on a top portion 103 of a substrate 7 outside the groove. This means that the sputtering method provides poor coverage. FIG. 5 also shows that a film is deposited on the side 101 of the groove. Poor coverage and film formation on the side of a groove adversely affect film formation on a substrate.
Below is described a magneto-optical disk of a magnetic domain wall displacement type, which is disclosed in Japanese Patent Application-Open No. 6-290496. Grooves which are concentrically formed in conventional magneto-optical disks and compact disks are not used to record information. However, because information is recorded also on the bottom of a groove in a recording medium of a magnetic domain wall displacement type, a functional film must be formed on the bottom similarly as on the flat parts (hereinafter, referred to as xe2x80x9clandsxe2x80x9d) of the medium outside the groove. In addition, the medium must be adapted so that no magneto-optical signal is produced from the side of the groove, which separates the bottom of the groove and the lands, to prevent interferences between the groove and the lands, to do so, a deposition amount of a film on the side of the groove must be minimized. That is, a recording medium of a magnetic domain wall displacement type requires film formation which is highly directional and has a high bottom coverage ration. The bottom coverage ratio is defined as the ration of a film formation rate on the bottom surface of the groove to a film formation rate on a surface outside the groove. The bottom coverage ratio can be obtained by the formula tA/tBxc3x97100 (%), wherein tA is the thickness of a film formed on the bottom surface of a groove and tB is the thickness of a film formed on surface outside the groove (see FIG. 4).
Conventional film forming methods which provide a high bottom coverage ratio include the low-pressure remote sputtering method, collimate sputtering method, and a high-frequency plasma assisted sputtering method which is disclosed in Japanese Patent Application Laid-Open No. 10-259480.
The low-pressure remote sputtering method allows sputtering particles to fly straight without scattering because the method uses a lower pressure and a longer mean free path than ordinary sputtering methods. The low-pressure remote sputtering method is also adapted to provide a longer distance between a target and a substrate and make particles fly at a right angle to the substrate.
The collimate sputtering method makes only sputtering particles flying at a right angle to a substrate to reach it and deposits the particles thereon by placing a cylinder having a plurality of holes made at a right angle to the substrate between a target and the substrate.
The high-frequency plasma assisted sputtering method allows flying sputtering particles to deposite by ionizing them in a space of plasma, which is produced near a substrate by applying a high-frequency voltage to the substrate, and directing the ionized sputtering particles at a right angle to the substrate, using a negative voltage (self bias) produced on the substrate due to plasma.
However, the low-pressure remote sputtering method is said to be limited to substrates with a groove aspect ratio up to about 4 in mass production because of its low film formation rate and low raw-material (target) use efficiency due to the long distance between the target and substrate.
The collimate sputtering method has a problem of low film formation rate and low raw-material use efficiency due to a loss caused by sputtering particles deposited on the collimator, and is limited to substrates with a groove aspect ratio up to about 3.
The high-frequency plasma assisted sputtering method can be used for substrates with a groove aspect ratio of 4 or more. However, the sputtering method allows charged particles in plasma to penetrate a substrate, thus heating it because plasma is produced by applying a high-frequency voltage to the substrate. Thus in the sputtering method, it is difficult to form a film on a substrate made of a material which has low heat resistance, such as resin used as the substrate material for recording media, including compact disks and magneto-optical disks.
It is an object of the present invention, intended to solve the above-described problems, to provide a film forming method and film forming apparatus which can be used to form a film at a high bottom coverage ratio even on a substrate with deep grooves on its surface.
It is another object of the present invention to provide a film forming method and a film forming apparatus which can prevent substrate temperature from increasing.
It is still another object of the present invention to provide an ionization film forming method and an ionization film forming apparatus which promote discharge gas excitation and ionization, thereby increasing the efficiency of ionization of evaporated particles.
These objects are attained by a method for forming a deposited film by sputtering, comprising the steps of:
ionizing sputtering particles and applying a periodically changing voltage to an electrode provided near a substrate,
wherein a time for which a voltage equal to or larger than an intermediate value between maximum and minimum values of the above-described voltage is applied is shorter than a time for which a voltage equal to or smaller than the intermediate value is applied.
The objects are also attained by an ionization sputtering apparatus for forming a deposited film by directing sputtering particles to a substrate using an electric field produced near the substrate, comprising:
a sputtering chamber with an evacuating system;
gas introducing means for introducing a processing gas into the sputtering chamber;
a target placed in the sputtering chamber;
ionizing means provided between the target and the substrate;
an electrode disposed near the substrate; and
voltage applying means for applying a periodically changing voltage to the electrode under such a condition that the time for which a voltage equal to or larger than an intermediate value between maximum and minimum values of the applied voltage is applied is shorter than the time for which a voltage equal to or smaller than the intermediate value is applied. Detailed descriptions will be given later with reference to examples.